1. Field of the invention
The present invention relates to electrochemical technologies and more in particular to methods and electrolysis cells for producing halogens by the electrolysis of aqueous solution of halides.
2. State of the art
One of the most significative innovations in this field of technology has been the adoption of hydraulically impervious, ion exchange membranes as separators between respective anodic and cathodic compartments of electrolysis cells in place of the traditional microporous asbestos diaphragms. These membranes are almost exclusively formed by at least a thin film of a perfluorinated polymer containing pendant polar groups attached to the polymeric matrix.
Another recent innovation, made possible in a way by the novel nature of the separators, has been the development of so-called "O-gap" electrolysis cells, i.e. of cells wherein the electrodes, necessarily porous, are held in direct contact with the surface of the separating element of the cell, that is of the membrane. Such a disposition of the electrodes implies, among others, the advantages of minimizing the cell voltage (ohmic drop), by minimizing the path traversed by the ionic current, together with an intrinsic ease of ensuring an almost perfect distribution of the current density across the whole cell surface, by freeing said requisite from the limitations which have always been imposed by fabrication tolerances of such large surface structures as metal screen electrodes welded on a series of current distributing metal supports.
An early electrolysis cell of this kind is described in the U.S. Reissue Pat. No. Re. 32,077. In this patent the concept of a O-gap membrane cell is exemplified by a monopolar cell containing a plurality of tubular screen anodes, cladded over their external surface by an ion exchange membrane and wherein the cathode is formed by a mass of packing material (static or fixed bed) filling completely the space between the internal walls of a cathodically polarized container of the cell and the membrane cladded surface of the tubolar anodes which traverse the cathodic compartment. Such a way of implementing an O-gap cell, though offering the advantage of an extremely easy construction, has not found widespread commercial application primarily because in producing chlorine and caustic soda in a cell equipped with a modern ion exchange membrane, the catholyte (caustic soda at 30-35% by weight) has a high density and viscosity, and the hydrogen developed on the cathode surface as minute bubbles disperses in the viscous liquid forming a gas-liquid dispersion rather difficult to separate because of the ability of hydrogen in such minute bubbles to disperse itself readily within the bulk of the liquid catholyte. In a cathodic compartment completely filled with such a packing material, the total volume of interstices among the grains or fibers of the packing material is only a small percentage of the total volume of the cathodic compartment and such a constriction of the flow of the hydrogen -catholyte dispersion through the cahodic compartment (normally from the bottom of the compartment to an outlet port placed in the top portion of the compartment) further reduces the possibilities of a fast removal of the gas from the cathodic compartment. This determines a greater than otherwise normal volumetric percentage of gas in the bulk of the catholyte within the cathodic compartment (i.e. within the catholyte traversed by flux lines of ionic electric current). This fact is notably negative as it tends to increase the ohmic drop (cell voltage).
In order to overcome these limitations, a large number of ion exchange membrane cell configurations have been proposed, wherein the desired O-gap condition is obtained by pressing more or less uniformly by resilient means such as springs or other similar elastic structures, a screen electrodic structure (normally a cathode but in some cases an anode) against the surface of the separating membrane of the cell, which membrane being commonly flexible is pushed against a restraining surface provided by the cell counterelectrode which has a substantially rigid nature. U.S. Pat. Nos. 4,340,452; 4,530,743; 4,444,632; and 4,536,263 are some examples of as many O-gap cells of this kind.
The quest of a cell configuration capable of associating the advantages of the O-gap configuration to the utilization of essentially flat screen electrode structures which leave empty the remaining space of the electrodic compartment for favoring a fast fluid circulation in and out of the compartment, besides allowing the construction of modular filter press type cells, has lead to the development of cell structures wherein the problems of satisfying stringent fabrication tolerances in terms of planarity, parallelism of structures, have re-presented themselves in order to prevent "pinching" of the membrane at points more stressed than others, and more in general have led to complex and costly cells.